The Toyota Prius has performed badly in a new test by the US Insurance Institute for Highway Safety (IIHS). Most US midsize cars are already achieving reasonable results in the small overlap crash test. but the Toyota Prius received a poor safety rating.

The test shows what happens when the front corner of a car clips another vehicle or an object like a tree. Of the 18 midsize cars tested, 13 were rated either good or acceptable, three were rated marginal, and two were poor, including the Toyota Prius. Commenting on the performance of the Prius, the IIHS said: “The Prius’ main crush-zone structure is concentrated in the middle half of the car’s front end. The car illustrates what can go wrong in a 25 percent overlap crash, despite otherwise good safety ratings.”

In a lot of cars, the 25 percent overlap frontal impact doesn’t connect with the main crash structures. Instead the forces go directly into the wheel, suspension system and firewall, challenging the vehicle’s structural integrity and risking serious injuries.

The IIHS tested luxury and premium midsize cars earlier this year with disappointing results. Only three of 11 cars got good or acceptable scores. The Volvo S60 was the best, partly because the company has been doing this test in-house for more than 20 years. Others, including the Lexus IS, Mercedes C-Class Audi A4 and Volkswagen CC did badly. The VW even lost a door in the crash.

The small overlap test focuses the attention of car designers and engineers on two key factors. It makes them look at the vehicle structure and it encourages bigger airbags that cover the A-pillars. The vehicle rotates a lot in the crash, sending the occupant off the side of a normal airbag, hitting the inside of the vehicle.

It’s unlikely that the tests will be introduced for European vehicles. Accident researchers and safety experts believe other tests should be prioritised instead. They want to promote the fitment of adaptive airbags and seatbelts – analysis of EU accident data indicates that this would do more to reduce serious injuries and deaths. Related story: Safety researcher calls for europe to adopt different crash tests to the US

A UN Working Group is looking at improved regulations for frontal impact and is thought to favour a full-width test instead. Euro NCAP also has a group working on this issue.

Small overlap crash test results for small SUVs are expected in the spring.